Fast protection scheme for passive optical network

ABSTRACT

The present invention relates to a method and devices for fast protection of an optical network system, in particular for a Passive Optical Network (PON), such as a Gigabit-capable Passive Optical Network (GPON). In the method, it is detected that the communication from a first optical network device is lost. Switching of functionality is initiated from a first optical line termination device to a second optical line termination device, and a control message is sent from the second optical line termination device to the first optical network device such that the first optical network device is prevented from moving into initial state. Furthermore, the method comprises determining and setting timing settings for the first optical network device.

TECHNICAL FIELD

The present invention relates to a method and devices for protection ofan optical network system, in particular for a Passive Optical Network(PON), such as a Gigabit-capable Passive Optical Network (GPON).

BACKGROUND

In recent years, the requirement for data transfer capacity and reliablenetworks have increased. Standards, such as ITU-T G.984.1, ITU-TG.984.2, and ITU-T G.984.3, have been developed in order to increase thespeed and accordingly the capacity of optical network systems.

Reliability of communication networks is an increasingly importantparameter, and accordingly continuous operation of optical networks incase of breakage of fibers or malfunctioning nodes or devices isdesired. The ITU-T standard specifying GPON includes four differentprotection switching possibilities in ITU-T G.984.1 (03/2003). Theseare:

-   -   Fiber duplex system (type-A scheme): Only the trunk fiber is        duplex. In case of a feeder fiber break, the spare fiber can be        switched in manually. Since the switching should be automated in        larger networks, fiber switches are necessary which are costly        and render this scheme uneconomical.    -   OLT-only duplex system (type-B scheme): Trunk fiber and optical        line termination (OLT) line terminal (LT) are duplex. One OLT-LT        is in operation, the other is in hot-standby, kicking in if the        OLT-LT fails or if the trunk breaks. Optical Network Unit        (ONU)/Optical Network Termination (ONT) and drop fibers are        simplex. Since only the components that are shared by the users        are duplex, the scheme shows a good tradeoff between costs and        fault tolerance.    -   Full duplex system (type-C scheme): Fully failure tolerant        system, since all components are duplex. The whole distribution        fiber network has to be doubled, leading to very high costs for        this solution.    -   Partial duplex system (type-D scheme): With a mix of type-B and        type-C protection, simplex and duplex users can be mixed on the        PON. The scheme proposed in the standard is unworkable due to        the fiber-cross in the splitter.

So far, the interest in protection switching in the GPON community hasbeen limited since typical deployment scenarios support a maximum of 64split on 20 km reach, i.e. the protection benefits are small compared tothe costs of the schemes. However, with the development ofreach-extended systems with higher splits (128 to 256) protection willbecome an essential part of the PON system, since a trunk fiber cut orOLT failure will cause service outage for a high number of users.

Considering a type-B scheme, switch-over based on the standard procedureas proposed in the standard will take several minutes to occur since allONUs/ONTs move to initial state where a full initialization includingconfiguration, activation and ranging is necessary. Thus such schemescannot recover quickly and connection or session continuity cannot beaccomplished.

SUMMARY

Accordingly, it is an object of the present invention to provide amethod and devices for minimizing downtime of a PON in case of fiberfault or device failure in the network.

A method for protecting an optical network system is provided. Theoptical network system comprises a first optical network device, anoptical distribution network, and a first and second optical linetermination device, wherein the first optical network devicecommunicates with the first optical line termination device on a firstconnection via the optical distribution network and a first opticalfiber trunk connected to the first optical line termination device.Further, the first optical network device has a second connection to thesecond optical line termination device via the optical distributionnetwork and a second optical fiber trunk connected to the second opticalline termination device. In the method, it is detected that thecommunication from the first optical network device is lost. Switchingof functionality is initiated from the first optical line terminationdevice to the second optical line termination device, and a controlmessage is sent from the second optical line termination device to thefirst optical network device such that the first optical network deviceis prevented from moving into initial state. Furthermore, the methodcomprises determining and setting timing settings for the first opticalnetwork device.

Furthermore, an optical line termination device for an optical networksystem comprising a first optical network device and an opticaldistribution network is provided. The optical line termination devicecomprises an optical fiber trunk interface and being adapted tocommunicate with the first optical network device on a connection viathe optical distribution network and an optical fiber trunk.Additionally, the optical line termination device comprises a controllerdevice connected to the optical fiber trunk interface and being adaptedto detect that the communication from the first optical network deviceis lost. When the optical line termination device detects thatcommunication is lost, the optical line termination device is adapted toinitiate switching of functionality from another optical linetermination device to the optical line termination device and send acontrol message to the first optical network device such that the firstoptical network device is prevented from moving into initial state.Furthermore, the optical line termination device is adapted to determineand set timing settings for the first optical network device.

It is an important advantage of the present invention that an opticalnetwork device is prevented from moving into initial state when thefirst optical fiber trunk and/or the first optical line terminationdevice fail or break down. Hereby, the downtime of the optical networksystem is reduced considerably by avoiding complete re-initialization ofthe optical network devices that are hosted by or connected to the firstoptical line termination device.

An optical line termination system is provided, comprising a firstoptical line termination device and a second optical line terminationdevice. The second optical line termination device is an optical linetermination device as described herein.

It is an important advantage of the present invention that sessionand/or connection continuity is provided or maintained in a PON, such asa GPON, in case of fiber fault of the duplex fiber or failure of duplexnetwork components, such as an OLT.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become readily apparent to those skilled in the art by thefollowing detailed description thereof, in particular by detaileddescription of exemplary embodiments thereof with reference to theaccompanying drawings, in which:

FIG. 1 schematically illustrates an optical network system employing themethod according to the present invention,

FIG. 2 schematically shows a block diagram of an embodiment of anoptical line termination device according to the present invention,

FIG. 3 schematically shows a block diagram of another embodiment of anoptical line termination device according to the present invention,

FIGS. 4-6 show different embodiments of an optical line terminationsystem according to the invention,

FIG. 7 schematically shows a flow diagram of an embodiment of the methodaccording to the present invention,

FIG. 8 schematically shows a flow diagram of an embodiment of the methodaccording to the present invention,

FIG. 9 schematically illustrates a flow diagram of an embodiment ofselected steps of the method according to the present invention, and

FIG. 10 schematically illustrates a flow diagram of an embodiment ofselected steps of the method according to the present invention.

DETAILED DESCRIPTION

The figures are schematic and simplified for clarity, and they merelyshow details which are essential to the understanding of the invention,while other details have been left out. Throughout, the same referencenumerals are used for identical or corresponding parts or features.

FIG. 1 illustrates an optical network system 100 implementing the methodaccording to the present invention. The optical network system operatesin a type-B protection scheme scenario as described in ITU-T G.984.1 andcomprises a first optical network device (ONU₁) 102, an opticaldistribution network 104 comprising a splitter 105, a first optical linetermination device (OLT₁) 106 and second optical line termination device(OLT₂) 108. The first optical network device 102 communicates with thefirst optical line termination device 106 on a first connection via theoptical distribution network 104 and a first optical fiber trunk 110connected to the first optical line termination device 106. Furthermore,the first optical network device 102 has a second connection to thesecond optical line termination device 108 via the optical distributionnetwork 104 and a second optical fiber trunk 112 connected to the secondoptical line termination device 108. OLT₁ communicates with OLT₂ on afirst control connection 116. The first control connection may be anelectrical and/or optical connection. The first control connection maybe wireless.

The first optical line termination device 106 and the second opticalline termination device 108 are configured in a duplex configuration andis connected to a number N of optical network devices (ONU₁, ONU₂, . . ., ONU_(N)). Typically, N=2^(p), where p equals 0, 1, 2, 3, 4, 5, 6, 7,8, 9, 10 corresponding to N equal to 1, 2, 4, 8, 16, 32, 64, 128, 256,512, 1024. A higher number of optical network devices may becontemplated.

The splitter 105 comprises N ONU drop ports towards the N opticalnetwork devices and two trunk ports connected to OLT₁ and OLT₂,respectively.

In regular operation of the optical network system 100, OLT₁ is activeand serves the optical network devices ONU₁, ONU₂, . . . , ONU_(N). Thesecond optical line termination device OLT₂ is in hot standby, i.e.listens to upstream traffic on the second connection (second opticalfiber trunk 112).

In case OLT₁ or the first optical fiber trunk 110 fails, thecommunication between OLT₁ and ONU₁-ONU_(N) will stop immediately, whichwill be detected immediately by all active ONUs. OLT₂ is adapted todetect that communication from one or more optical network devices isnot present or lost. In an embodiment, the lost communication isinterpreted as a failure or fault in OLT₁ or in the first optical fibertrunk 110 and switching of functionality from OLT₁ to OLT₂ is initiated.OLT₂ immediately, e.g. within 50 milliseconds from detection ofcommunication loss, sends a control message, e.g. a broadcast POPUPmessage, to all ONUs in order to prevent ONUs to enter init state.Bringing ONUs back to operating state from the init state is timeconsuming and should therefore be avoided. In general, it is importantthat the ONUs receive the control message before expiry of timer T2 (100ms). Subsequently, the ONUs are ranged by the second optical linetermination device OLT₂.

As mentioned above, an optical line termination device for an opticalnetwork system is provided. In accordance with the method, the opticalline termination device may be adapted to determine the cause ofcommunication loss from the first optical network device and initiateswitching of functionality from the other optical line terminationdevice based on the determined cause of communication loss.

The optical line termination device according to the invention maycomprise a first interface connected to the controller device. The firstinterface may be connected to the other optical line termination device,e.g. the first optical line termination device, via a first controlconnection. Hereby, the optical line termination device may be adaptedto exchange information with the other optical line termination device.The exchanged information may comprise status information about theother optical line termination device and/or the optical fiber trunkconnected to the other optical line termination device.

In order to secure information exchange with the other optical linetermination device, the control connection may be duplex. Accordingly,the optical line termination device may comprise a second interfaceconnected to the controller device and may be adapted to exchangeinformation with the other optical line termination device via thesecond interface on a second control connection in case of failure ofthe first control connection.

The optical line termination device may be adapted to send a controlmessage to and determine and set timing settings for each or selectedones of the optical network devices connected to the optical linetermination device. Accordingly, the optical line termination device maybe adapted to send a control message to a second optical network devicein the optical network system and determine and set timing settings forthe second optical network device.

The optical line termination device may, e.g. when implemented oradapted for a PON according to ITU-T G.984.1, ITU-T G.984.2, and/orITU-T G.984.3, be adapted to send a control message comprising abroadcast POPUP message to the optical network devices connected to theoptical line termination device.

Determination and setting of timing settings for the optical networkdevices connected to the optical line termination device according tothe invention may be implemented in a number of ways. In an embodiment,the optical line termination device may be adapted to send a rangingrequest message to the first optical network device and adapted toreceive a ranging response message from the first optical networkdevice.

The optical line termination device may be adapted to sent the rangingrequest message to one or more, e.g. only one or each, of the opticalnetwork devices, e.g. to the first and/or second optical network device,connected to the second optical line termination device. In anembodiment of the present invention, the ranging request message is sentto each of the optical network devices connected to the second opticalline termination device.

The optical line termination device may be adapted to retrieve thesecond set of timing settings for the optical network devices includingtiming setting for the first and/or second optical network device from adata storage, e.g. a data storage in the optical line terminationdevice. Alternatively or in combination, the second set of timingsettings or a part of the second set of timing settings may becalculated. For example, timing settings for an optical network device,e.g. the first optical network device, may be measured and the new(second set of) timing settings for the remaining optical networkdevices may be calculated based on the performed measurement and thefirst set of timing settings, e.g. retrieved from a data storage.

The optical line termination device may be adapted to set the timingsettings, e.g. including equalization delay, by sending a ranging timemessage. The ranging time message may be a broadcast ranging timemessage comprising a list of timing settings for the optical networkdevices connected to the optical line termination device.

The optical network device may be an Optical Network Unit (ONU) or andOptical Network Termination (ONT), e.g. in accordance with ITU-TG.984.3. The optical line termination device may be implemented inOptical Line Termination (OLT) e.g. in accordance with ITU-T G.984.3, oran Optical Line Termination (OLT) Line Terminal (LT).

The optical line termination device(s) may comprise a core interfacetowards the backplane network.

FIG. 2 illustrates an embodiment of the optical line termination deviceaccording to the present invention. The optical line termination device120 comprises a fiber interface 122, e.g. an optical fiber trunkinterface, a controller 124, a first interface 126 for exchanginginformation with another optical line termination device, e.g. on thefirst control connection 116, and a memory or data storage 128. Theoptical line termination device 120, e.g. implemented in the secondoptical line termination device 108, is adapted to detect that thecommunication from one or more of the optical network devices, e.g. thefirst optical network device is lost and initiate switching offunctionality from another optical line termination device, e.g. thefirst optical line termination device 106, to the optical linetermination device 120. Further, the optical line termination device 120is adapted to send a broadcast POPUP message to the first opticalnetwork device and determine and set timing settings for the firstoptical network device, e.g. as described in connection with FIG. 7 andFIG. 8. Further, the optical line termination device 120 may be adaptedto store network data such as round trip delays or other configurationdata from another optical line termination device, e.g. OLT₁ 106, in thememory 128. The memory 128 may be updated continuously, or with acertain frequency, via the first interface 126 and/or the secondinterface 130 with data from the other or first, optical linetermination device in order to facilitate a fast switchover offunctionality.

The condition of upstream silence on the optical line termination device108, 120 may not be sufficient to declare that the other optical linetermination device, e.g. OLT₁, or the first optical fiber trunk hasfailed. It is also possible that the system has been put out ofoperation or that ONUs are just silent for some time. In order to avoidunnecessary switching of functionality, the optical line terminationdevice 120 is adapted to determine the cause of communication loss fromthe first optical network device and initiate switching of functionalityfrom the other optical line termination device based on the determinedcause of communication loss. The cause of communication loss may bedetermined by detecting a status signal from the other, i.e. first,optical line termination device on the first control connection. In casethe status signal is absent, switching is initiated, control message issent, and timing settings are determined and set accordingly.

FIG. 3 illustrates an embodiment of the optical line termination device120′ further having a second interface 130 for a second controlconnection to the other optical line termination device. The opticalline termination device is adapted to exchange information with theother optical line termination device, e.g. OLT₁, via the secondinterface 130 in case of failure of the first control connection.

FIG. 4 illustrates an embodiment of an optical line termination systemaccording to the invention. The optical line termination system 150 hasa housing 151 comprises a first optical line termination device 106 anda second optical line termination device 120′. The first and secondoptical line termination devices 106, 120′ have first and second fiberinterfaces 122′ and 122, respectively, first interfaces 126 and 126′,respectively, and second interfaces 130 and 130′, respectively. Theinterfaces 126, 126′, 130, 130′ and corresponding first and secondcontrol connections 116, 152 allow OLT₂ to determine cause ofcommunication loss. Further, OLT₂ may be adapted to mirror networkrelated data (GPON and service definitions) from OLT₁ or vice versa,which may be important in order to perform switch over without loosingthe communication sessions in the optical network system.

FIG. 5 illustrates an embodiment of an optical line termination systemaccording to the invention. The optical line termination system 160 isimplemented in an Application Specific Integrated Circuit (ASIC) or in aField-Programmable Gate Array (FPGA) on a silicon board 161 andcomprises a first optical line termination device (OLT₁) 106 and asecond optical line termination device (OLT₂) 120. The first and secondoptical line termination devices 106, 120 have first and second opticalfiber trunk interfaces 122′ and 122, respectively. OLT₂ is adapted tomirror network related data (GPON and service definitions) from OLT₁ orvice versa via bus 162 which may be important in order to perform switchover without loosing the communication sessions in the optical networksystem. OLT₁ and OLT₂ may have shared data storage or separate datastorages.

The implementation of the optical line termination system illustrated inFIGS. 4 and 5 may be referred to as type B protection scheme with singlehoming.

FIG. 6 illustrates an embodiment of an optical line termination systemaccording to the invention. The optical line termination system 170comprises a first optical line termination device (OLT₁) 106 and asecond optical line termination device (OLT₂) 120.

The first 106 and second 108, 120 optical line termination devices havefirst and second optical fiber trunk interfaces 122′ and 122,respectively. The first control connection 116 comprises an externalcommunication link 172, e.g. via a Local Area Network (LAN), betweenOLT₁ and OLT₂, which are positioned in different locations. This type ofprotection may be referred to as type B protection with dual homing.

FIG. 7 is a schematic flow diagram illustrating an embodiment of themethod according to the invention. The method may be implemented in anoptical network system 100 illustrated in FIG. 1. In the method 202, itis detected in step 204 that the communication from the first opticalnetwork device is lost. The method 202 then proceeds to step 206 ofinitiating switching of functionality from the first optical linetermination device 106 to the second optical line termination device108. Subsequently, the method proceeds to step 208 of sending a controlmessage in the form of a POPUP broadcast message from the second opticalline termination device 108 to the first optical network device 102. Bysending a control message to the first optical network device, the firstoptical network device is prevented from moving into init state therebysaving valuable time for re-initialization of the PON. After sending thecontrol message in step 208, the method proceeds to step 210 ofdetermining and setting timing settings for the first optical networkdevice. The round trip delay for the first line termination device andthe second line termination device are different. The difference inround trip delay is caused by a difference in trunk delay d₁ for thefirst optical fiber trunk, and trunk delay d₂ for the second opticalfiber trunk.

In the method according to the invention, the step of sending a controlmessage may comprise sending any control message that prevents theoptical network devices, such as optical network units or opticalnetwork terminals, to move into initial state. Preferably, the step ofsending a control message comprises sending a broadcast POPUP message.In a GPON, it is important that the broadcast POPUP message reaches theoptical network devices before they move into initial state, i.e. beforetimer T2 expiry (100 ms).

The change in delay is important, since ONUs need to time their upstreambursts in such a way that all burst are received aligned at the OLTwithout temporal overlap (no collision). The OLTs are granting timeslotsfor upstream transmission to the ONUs. In case OLT₂ takes over, the onlything that changes is the trunk delay. In case the equalization delayEqD₁(n) for OLT₁ is replaced by the equalization delay EqD₂(n) for OLT₂in the equalization delay memory of each ONU_(n) (n=1, 2, . . . , N),the optical network system can be brought back into operation withoutloosing the whole optical network configuration.

Step 210 of determining and setting timing settings will be described inmore detail with reference to the embodiments of steps 208 and 210illustrated in FIGS. 9 and 10.

It may be desired to avoid unnecessary switching of functionality fromthe first optical line termination device to the second optical linetermination device, e.g. in case the loss of communication is caused byan outage or failure in the optical distribution network or other partsof the system. Accordingly, the method according to the presentinvention may comprise determining the cause of communication loss fromthe first optical network device and initiate switching of functionalitybased on the determined cause of communication loss. In an embodimentswitching may be initiated according to a switching scheme, e.g. only incase of failure or breakdown of the first optical fiber trunk and/or thefirst optical line termination device.

In an embodiment of the present invention, initiating switching offunctionality comprises exchanging information between the first opticalline termination device and the second optical line termination deviceon a first control connection, e.g. via a first interface in each of theoptical line termination devices. Information between the first opticalline termination device and the second optical line termination devicemay be exchanged on a second control connection in case of failure ofthe first control connection, i.e. the control connection may be duplex.The information may comprise timing settings, such as equalizationdelays for optical network devices connected to the first and secondoptical line termination device.

The optical network system may comprise a second optical network devicecommunicating with the first optical line termination device and thesecond optical line termination device via the optical distributionnetwork and the first and second optical fiber trunks, respectively. Inthat case, the method may further comprise sending a control messagefrom the second optical line termination device to the second opticalnetwork device and determining and setting timing settings for thesecond optical network device.

The timing settings for the optical network devices, e.g. theequalization delay settings for the optical network devices connected tothe first and second line termination devices, are often dependent onwhich optical line termination device is in operation, i.e. a first setof timing settings applies to the optical network devices when the firstoptical line termination device is in control and a second set of timingsettings applies to the optical network devices when the second opticalline termination device is in control.

In case the second optical line termination device takes over, thetiming settings of the optical network devices must be adjusted to thenew operation scheme. The timing settings, e.g. the second set of timingsettings, may be determined by sending, e.g. from the second opticalline termination device, a ranging request message and receiving aranging response message, e.g. from the first optical network device.

The ranging request message may be sent to one or more of the opticalnetwork devices, e.g. to the first and/or second optical network device,connected to the second optical line termination device. In anembodiment of the present invention, the ranging request message is sentto each of the optical network devices connected to the second opticalline termination device.

In an embodiment of the present invention, the second set of timingsettings for the optical network devices including timing setting, e.g.equalization delay, for the first and/or second optical network devicemay be partly or fully retrieved from a data storage. Alternatively orin combination, the timing settings or a part of the timing settings maybe calculated, e.g. timing settings for an optical network device may bemeasured and the new (second set of) timing settings for the remainingoptical network devices may be calculated based on the performedmeasurement and the first set of timing settings that may be retrievedfrom a memory, e.g. in the second line termination device.

The timing settings may be set by sending a ranging time message. Theranging time message may be a broadcast ranging time message comprisinga list of timing settings for the optical network devices.

FIG. 8 is a schematic flow diagram illustrating an embodiment of themethod according to the invention. The method 202′ may be implemented ina optical network system 100 illustrated in FIG. 1. In addition to thesteps 204, 206, 208 and 210 which are described in connection with FIG.7, the method 202′ comprises the step 212 of determining, after step 204of detecting loss of communication, if switching is to be initiated. Thedecision in step 212 comprises determining the cause of communicationloss, i.e. whether OLT₁ or the first optical fiber trunk has failed. Incase OLT₁ or the first optical fiber trunk has failed, the method 202′proceeds to step 206.

FIG. 9 schematically illustrates a flow diagram of exemplary embodimentsof steps 208 and 210 of the method according to the invention, e.g. themethods illustrated in FIGS. 7 and 8. The step 208 of sending a controlmessage to the first optical network device comprises sending abroadcast POPUP message. The broadcast POPUP message is sent from thesecond optical line termination device via the second optical fibertrunk to all optical network devices (ONU₁-ONU_(N)), including the firstoptical network device and the second optical network device if present,hosted by or connected to the first and second optical line terminationdevices. In the illustrated embodiment, step 210 of determining andsetting timing settings comprises ranging the ONUs one by one andsending a ranging time message to ONU_(n) for all n=1, . . . , N. Step210 comprises in step 250 and 252 selecting the first optical networkdevice ONU₁ and sending a ranging request message to the first opticalnetwork device. In step 254, the second optical line termination device108, 120 receives the ranging response message from the first opticalnetwork device thereby measuring the round trip delay RTD₂(1) betweenthe first optical network device and the second optical line terminationdevice. After step 254, the equalization delay EqD₂(1) for the firstoptical network device is calculated in step 256.

The steps 252, 254, and 256 are repeated until all round trip delaysRTD₂(n) and equalization delays RTD₂(n) for optical network devicesONU_(n) (n=1, . . . , N) connected to or hosted by the first and secondoptical line termination device are determined.

In an embodiment of the present invention, the equalization delaysEqD₂(n) for ONU_(n) (n=1, 2, . . . , N) are given by:EqD ₂(n)=EqD ₁(n)+Δd(n),where Δd(n) is the difference between the trunk delay d₂ for the secondoptical fiber trunk and the trunk delay d₁ for the first optical fibertrunk:Δd(n)=d ₂ −d ₁=RTD₂(n)−RTD₁(n),where RTD₁(n) is the round trip delay between OLT₁ and ONU_(n). Valuesfor EqD₁(n) and RTD₁(n) are retrieved from a memory, e.g. memory 128 inthe second optical line termination device 120, 120′.

Then the method proceeds to step 258 of setting the new timing settings,e.g. as illustrated by sending a Ranging Time message. The Ranging Timemessage may be a broadcast Ranging Time message that is sent to allONU_(n) (n=1, . . . , N) with a list of equalization delays for eachoptical network device. Step 258 may comprise sending N Ranging Timemessages, one for each optical network device. When the Ranging Timemessage is received, the optical network devices move back to operationstate and communication is again established.

FIG. 10 schematically illustrates a flow diagram of exemplaryembodiments of steps 208 and 210 of the method according to theinvention, e.g. the methods illustrated in FIGS. 7 and 8. Step 210comprises step 260 of selecting an optical network device ONU_(i) amongthe N optical network devices and step 252 of sending a ranging requestmessage to the selected optical network device ONU_(i). In step 254, thesecond optical line termination device, e.g. the second optical linetermination device 108, 120, 120′, receives the ranging response messagefrom the selected optical network device ONU_(i) thereby measuring theround trip delay RTD₂(i) between the selected optical network deviceONU_(i) and the second optical line termination device. In theembodiment illustrated in FIG. 10, the second optical line terminationOLT₂ only sends Ranging Request to a selected ONU_(i) (252) and performsa round trip measurement for the selected ONU_(i) (254). Then the methodproceeds to step 256, where the equalization delays EqD₂(n) for allONU_(n) (n=1, . . . , N) are calculated asEqD ₂(n)=EqD ₁(n)+Δd,where Δd is the difference between the trunk delay d₂ for the secondoptical fiber trunk and the trunk delay d₁ for the first optical fibertrunk and given by:Δd=d ₂ −d ₁=RTD₂−RTD₁,where RTD₂ is the measured round trip delay for the selected opticalnetwork device and RTD₁ is the round trip delay between the firstoptical line termination device and the selected optical network device.Delays apart from d₂ and d₁ have not changed. RTD₁ and/or EqD₁(n) may beretrieved from a memory in the second optical line termination device,e.g. memory 128. The new equalization delays EqD₂(n) are sent in step258, which is described in connection with FIG. 9.

It is an advantage of the method partly illustrated in FIG. 10 that thedetermination of the new equalization delays is performed on the basisof one round trip measurement, thereby saving N−1 round tripmeasurements.

In an embodiment, the equalization delays EqD₂(n) are determined duringinitial startup of the optical network and stored in a memory, e.g.memory 128. Accordingly, equalization delays EqD₂(n) may be retrievedfrom a memory. Thus in an embodiment of the present invention, steps252, 254, 256, and 260 may be replaced by the step of retrieving thetiming settings, e.g. EqD₂(n), from a memory.

The method, device and system according to the invention may beimplemented in any Passive Optical Network (PON), in particular in aGigabit-capable Passive Optical Network (GPON), e.g. as described inG.984.1-3, or in an Ethernet Passive Optical Network (EPON), e.g. asdescribed in IEEE 892.3ah.

It should be noted that in addition to the exemplary embodiments of theinvention shown in the accompanying drawings, the invention may beembodied in different forms and should not be construed as limited tothe embodiments set forth herein. Rather, the embodiments illustratedherein are provided so that this disclosure will be thorough andcomplete, and will fully convey the concept of the invention to thoseskilled in the art.

The invention claimed is:
 1. An optical network device for use in anoptical network system, said optical network system including a secondoptical network device, first and second optical line terminationdevices and an optical distribution network, wherein the optical networkdevice is adapted to communicate with the optical line terminationdevices via the optical distribution network and an optical fiber trunk,wherein the optical network device comprises a receiver operative to:receive a control message from the second optical line terminationdevice such that the optical network device is prevented from movinginto an initial state when switching of functionality from the firstoptical line termination device to the second optical line terminationdevice; and, receive, from the second optical line termination device,broadcast timing settings for at least said optical network device andthe second optical network device based on only one roundtrip delaymeasurement, which timing settings are based on measured timing settingsfor the second optical network device.
 2. The optical network deviceaccording to claim 1, wherein the receiver is further operative toreceive a ranging request message and to send a ranging responsemessage.
 3. The optical network device according to claim 1, wherein thereceiver is further operative to receive a ranging time message.
 4. Theoptical network device according to claim 1, wherein the timing settingscomprise equalization delay settings.
 5. A method in an optical networkdevice for an optical network system, said optical network systemincluding a second optical network device, first and second optical linetermination devices and an optical distribution network, wherein theoptical network device is adapted to communicate with the optical linetermination devices via the optical distribution network and an opticalfiber trunk, wherein the method comprises: receiving a control messagefrom the second optical line termination device such that the opticalnetwork device is prevented from moving into an initial state whenswitching of functionality from the first optical line terminationdevice to the second optical line termination device; and, receiving,from the second optical line termination device, broadcast timingsettings for at least said optical network device and the second opticalnetwork device based on only one roundtrip delay measurement, whichtiming settings are based on measured timing settings for the secondoptical network device.
 6. The optical network device according to claim5, wherein the timing settings comprise equalization delay settings.